Freeplay can be defined, but without limitation, as the positional or rotational variation in mechanical parts. All aircraft include movable control surfaces for directional control during flight. Control surfaces, which are movable, include, but without limitation, the rudder, the aileron, the elevator, and the all-moveable horizontal tail of an aircraft. These control surfaces, particularly those operated in a military environment must be monitored for the freeplay of the control system. Excessive freeplay can result in flutter, vibrations, fatigue, divergence, or other instabilities. Historically, freeplay testing involves using complex ground fixtures. This testing is time consuming, requires a large team, and is very expensive.
Freeplay is typically tested by applying a known load to the control surface and measuring the corresponding deflection. This is repeated over numerous different loads (both positive and negative) for a minimum of three cycles. For example, a rotational variable differential transformer is used to measure rotation between a rudder and a tail of an aircraft, while a test hydraulic actuator and a load cell are used to record the load. Such setups are labor intensive and take a significant amount of time.
The latest United States Navy program requires that a freeplay test be performed after every flight. This adds excessive cost and time to aircraft maintenance. Therefore, an alternative method to test freeplay of control surfaces is needed. In particular, there is a need to test the free play of a ruddervator. A ruddervator may be defined, but without limitation, as a pair of control surfaces on a v-tail design that perform the function of both a rudder and an elevator.